My father-in-law has baled hay on some crazy steep hills with his round baler... 300 to 400 bales a year, when my wife was a kid, they did 10 to 12 thousand square bales every summer with an old NH 243 hayliner.
I have yet to try baling our fields, be a few more years until the briars are beat back enough in the fields to yield good horse feed hay. The previous owner let the fields go a few years before we bought it 3 years ago.